In recent years, with significant advancement of computer technology and electronics technology, the technology of medical imaging diagnostic device is rapidly improved. Various medical imaging diagnostic devices with different physical photographic principles (e.g., an ultrasonic diagnostic device, an X-ray device, a CT device and an MRI diagnostic device) have been developed and used in disease diagnosis in the respective purposes. In images photographed with these medical diagnostic devices, speckle noise due to dispersion and interference of signal in an heterogeneous medium in a living body, shot noise due to graininess of a transmission signal, electrical noise caused inside the device and the like are superposed. Further, in some cases, a high frequency component supposed to be originally obtained on the border of a tissue cannot be sufficiently obtained since the frequency band of a transmission/reception signal is narrow or for some other reasons. This may blur edges included in the image.
The noise superposition and edge blurring degrade image quality and have an adverse effect upon diagnosis. To suppress the noise and edge blurring and accurately read a significant structure of a morbid portion or the like, application of noise removal processing and signal component enhancement processing by image processing, in addition to improvement in hardware performance and signal transmission/reception system, are desired.
As noise removal by image processing applied to medical imaging, processing using a liner filter, a median filter and wavelet transformation are well known (Patent Documents 1 and 2).
On the other hand, as widely used signal component enhancement processing, generally, processing using an edge enhancing space filter such as a Laplacian filter, procesing of extracting a high frequency component and amplifying the extracted high frequency component thereby performing signal enhancement, and the like, can be given (Patent Documents 3 to 6).
In the image quality improvement methods described in the Patent Documents 1 to 6, only one of the noise removal processing and the signal component enhancement processing is performed. However, in the image quality improvement methods disclosed in the Patent Documents 7 and 8, the both noise removal processing and the signal component enhancement processing are performed.
In the conventional signal component enhancement processing using a Laplacian filter and the method of amplifying a high frequency component, generally, ringing occurs around an edge, which often degrades the image quality. On the other hand, in recent years, edge enhancement processing using a morphological filter attracts attention as processing capable of enhancing a signal component in addition to suppression of ringing around an edge. The method disclosed in the Patent Document 1 is one of such methods.    Patent Document 1: Japanese Published Unexamined Patent Application No. Hei 3-100773    Patent Document 2: Japanese Published Unexamined Patent Application No. 2002-133399    Patent Document 3: Japanese Published Unexamined Patent Application No. Hei 9-62836    Patent Document 4: Japanese Published Unexamined Patent Application No. Hei 9-50522    Patent Document 5: Japanese Published Unexamined Patent Application No. Hei 9-248291    Patent Document 6: Japanese Published Unexamined Patent Application No. Hei 10-63836    Patent Document 7: Japanese Published Unexamined Patent Application No. 2005-296331    Patent Document 8: Japanese Published Unexamined Patent Application No. 2001-283215    Patent Document 9: Japanese Published Unexamined Patent Application No. 2002-253546    Non-Patent Document 1: J. G. M. Schavemaker et al.: Image Sharpening by Morphological Filtering, Pattern Recognition, Vol. 33, pp. 997-1012 (2000)    Non-Patent Document 2: Daniel J. Jobson et al.: The Statistics of Visual Representation, Visual Information Processing XI, Proc. SPIE 4736, (2002)